The Romans were superstitious that even numbers were unlucky, so their months were 29 or 31 days long

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When Rome emerged as a world power, the difficulties of making a
calendar were well known, but the Romans complicated their lives because
of their superstition that even numbers were unlucky. Hence their months
were 29 or 31 days long, with the exception of February, which had 28
days. However, four months of 31 days, seven months of 29 days, and one
month of 28 days added up to only 355 days. Therefore the Romans invented
an extra month called Mercedonius of 22 or 23 days. It was added every
second year.

Even with Mercedonius, the Roman calendar eventually became so far off
that Julius Caesar, advised by the astronomer Sosigenes, ordered a
sweeping reform. 46 B.C. was made 445 days long
by imperial decree, bringing the calendar back in step with the seasons.
Then the solar year (with the value of 365 days and 6 hours) was made the
basis of the calendar. The months were 30 or 31 days in length, and to
take care of the 6 hours, every fourth year was made a 366-day year.
Moreover, Caesar decreed the year began with the first of January, not
with the vernal equinox in late March.

This calendar was named the Julian calendar, after Julius
Caesar, and it continues to be used by Eastern Orthodox churches for
holiday calculations to this day. However, despite the correction, the
Julian calendar is still 111/2 minutes longer than the actual
solar year, and after a number of centuries, even 111/2 minutes
adds up.